Deployment of high speed transparent and reconfigurable optical networks requires effective flexible and robust Optical Performance Monitoring (OPM) techniques for ensuring high quality of service. The modern high speed networks are susceptible of optical signal degradations, mainly due to the Amplified Spontaneous Noise (ASE) from the optical amplifiers. Real time monitoring of the OSNR is a requirement in order to ensure the signal quality and in order to monitor potential failures in the transmission link.
The most common method to monitor the OSNR is based on the spectral analysis of the Transmission WDM signals and derives the OSNR by interpolating the out of band noise level into the signal band, namely by estimating the in-band noise level using the out of band noise level [D. C. Kilper, R. Bach, D. J. Blumental, D. Einstein, T. Landolsi, L. Olstar, M. preiss and A. E Willner, “Optical performance monitoring”, J. Lightwave. Technology., vol 22, no 1, pp 294-304, 2004]. However such a technique suffers from the use of optical filtering and routing in the link path since the out of band noise must be filtered out and therefore the interpolating method leads to severe underestimates of the real OSNR level.
Methods to derive OSNR level by estimating the in band noise level directly, even in the presence of optical filters in the link, are referred as “true OSNR” methods. Several true OSNR methods [for example, G. Rossi, T. E Dimmick and D. J Blumenthal, “Optical performance monitoring in reconfigurable WDM optical networks using subcarrier multiplexing”, J. Lightwave Technology, vol. 18, n12, pp 1639-1648, 2000] have been proposed and are based on various approaches such as electrical carrier to noise monitoring, polarization nulling, optical delay interferometer, nonlinear transfer functions using an optical parametric amplifier, a nonlinear loop mirror. Some of these methods are sensitive to other system impairments such as Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD). and this makes the OSNR monitoring more challenging. Relevant references are presented at the end of the description.
A method of True OSNR monitoring technique based on Stimulated Brillouin Scattering (SBS) effect [WO 2008151384 A1], has an advantage in that it is insensible to CD and PMD. The SBS effect [M. J Damzem, V. Vlad, A Mocofanescu, V. Badin, “Stimulated Brillouin Scattering: Fundamentals and Applications”, Institute of Physics, Series in Optics and Optoelectronics (CRC Press, 2003)] is a spectral nonlinear effect which leads to the nonlinear power transfer from the signal spectral component to a Stoke wave (down shifted in frequency with respect to the signal frequency) propagating in the backward direction with respect to the signal. The OSNR technique based on the SBS effect uses the fact that when a signal has its higher spectral components above the SBS threshold, the efficiency of the power being transferred to the Stoke wave is altered by the noise present within the signal band.
The noise being present within an optical signal in a real optical system (such as an optical link) is the so-called Amplifier Spontaneous Emission (ASE) noise being introduced by optical amplifiers which form part of the optical link. FIG. 1 of the WO 2008151384 is demonstrated as FIG. 1a (prior art) of the present patent application. FIG. 1a schematically illustrates the ASE noise, being always present within a real optical signal, as a noise source 22 which introduces a variable value of noise to a pure optical signal produced by an optical signal source 20. The resulting optical signal is then fed to an SBS based OSNR monitor 10.
WO 2008151384 further demonstrates results of the SBS based OSNR technique for 40 Gbps NRZ (Non return to Zero) OOK (On-Off Keying) signal, which presents dynamic OSNR monitoring range of 15 dB for OSNR from 15 to 30 dB. Quite high sensitivity (15 dB) is demonstrated due to the fact that the 40 Gbps NRZ OOK signal spectrum presents a prominent spectral peak at the carrier wavelength which is sufficiently narrow to stimulate an efficient SBS effect. The efficiency is also enhanced by the fact that the inventors of WO 2008151384 use a broadband bandpass filter (1 nm bandwidth) which, however, is not compliant with operations with 100 GHz and 50 Ghz channel spacing. Moreover, the NRZ OOK modulation format presents severe system penalties for bit rates of 40 to 100 Gbps; phase modulation formats are preferred and are optionally combined with polarization multiplexing scheme for additional CD and PMD impairment relaxations. Furthermore, with the network operating at 40 and 100 Gbps, and with utilizing the modulation formats such as DPSK (Differential phase shift keying), DQPSK (Differential Quaternary Phase Shift Keying) and DP-QPSK (Dual Polarization Quaternary Phase Shift Keying), OSNR requirements become stronger and the network links should be planned to meet OSNR of 15 dB and higher at the link end.
A real optical system such as a network link or the like, in order to be practically useful for carrying high bit rate optical signals, must have OSNR higher than 15 dB (i.e., must have a low in-band ASE noise level). Therefore, the optical signal carried through such an optical link cannot cause a significant change in the SBS induced reflected power. Due to that, the OSNR monitoring sensitivity range of the apparatus described in WO 2008151384 A1 will be drastically limited when applied to real modern optical systems. It should be further noted that when the channel grid in such optical systems is limited to 50 GHz, the amount of in band noise should be even more reduced. It means that for the spacing of 50 GHz, the sensitivity of the WO 2008151384 apparatus becomes totally unacceptable.
However, WO 2008151384 describes a set-up for measuring/monitoring OSNR in real optical systems (FIG. 4), which is reproduced in the present application as FIG. 1B. The optical signal 42 comprising in band noise is transmitted via an optical network link 44 and is tapped from it to the SBS based OSNR monitor 10.
Upon analyzing the sensitivity of the real system set-up of WO 2008151384 A1, that set-up occurs to be:                a) ineffective (having low sensitivity) for monitoring signals with relatively high OSNR which is the condition for optical links at high bit rates (i.e., 40 Gbps and higher);        b) practically inapplicable for modulation formats other than NRZ OOK, which are more preferable than NRZ OOK for the high bit rates.        
Therefore an OSNR monitoring technique is required, which would ensure a sufficient dynamic monitoring range for OSNRs ranging from 15 to 30 dB.